With an explosive increase in demand of a broadband multimedia communication service such as the Internet or a high-definition digital TV broadcast, a dense wavelength-division multiplexing optical fiber communication system, which is suitable for a long-distance and large-capacity transmission and is highly reliable, has been introduced in trunk line networks and metropolitan area networks. In access networks, an optical fiber access service spreads rapidly. In such an optical fiber communication system, cost reduction for laying optical fibers as optical transmission lines and improvement of spectral efficiency per optical fiber are important. Therefore, a wavelength-division multiplexing technology which multiplexes multiple optical signals having different wavelengths is widely used.
In an optical transmitter for such a high-capacity wavelength-division multiplexing communication system, an optical modulator is required. In the optical modulator, high speed operation with small wavelength dependence is indispensable. Further, an unwanted optical phase modulation component which degrades the waveform of the received optical signal after long-distance transmission (in the case of using optical intensity modulation as a modulation method), or an optical intensity modulation component (in the case of using optical phase modulation as a modulation method) should be suppressed as small as possible. A Mach-Zehnder (MZ) optical intensity modulator in which waveguide-type optical phase modulators are embedded into an optical waveguide-type MZ interferometer is suitable for such a use. In general, a currently used MZ optical intensity modulator is based on a so-called planar waveguide circuit in which titanium is diffused into the surface of a lithium niobate (LN: LiNbO3) substrate which is a typical electro-optic crystal having a refractive index that changes in proportion to an applied electric field. A typical MZ interferometer has a configuration in which waveguide-type optical phase modulator regions and optical waveguide-type multiplexer/demultiplexer regions are monolithically integrated on the same LN substrate. Further, electrodes for applying the electric field to the waveguide-type optical phase modulator are provided in the waveguide-type optical phase modulator.
To increase the transmission capacity per wavelength channel, a multilevel optical modulation signal system having a smaller optical modulation spectrum bandwidth than a typical binary light intensity modulation system is advantageous in terms of the spectrum use efficiency, wavelength dispersion of an optical fiber, and resistance to polarization mode dispersion, each of which poses a problem. This multilevel optical modulation signal system is considered to become mainstream particularly in optical fiber communication systems in trunk line networks exceeding 40 Gb/s, the demand for which is expected to increase in the future. For such use, a monolithically integrated multilevel optical modulator in which two MZ optical intensity modulators described above and an optical multiplexer/demultiplexer are used in combination has recently been developed. The LN-based MZ optical intensity modulator modules, which are currently commercially available, have some problems with the size (electrode length: about 5 cm, module length: about 15 cm), the driving voltage (about 5 Vp-p), and the like. However, since there is no practical optical modulator which surpasses the LN-based MZ optical intensity modulator in high-speed long distance optical transmission properties, it is still widely used for an optical transmitter unit or the like in various optical communication systems in trunk line networks.
In high speed optical modulation by using this optical modulator, especially in the high-frequency region in which the frequency of the modulation electric signal is over 1 GHz, the propagating wavelength of the modulation electric signal becomes equal to or shorter than the length of the electrode serving as means for applying an electric field to the optical phase modulator region in the LN-based optical modulator. Therefore, voltage distribution of the electrode is no longer regarded as uniform in an optical signal propagation axis direction. To estimate optical modulation characteristic exactly, it is required to treat the electrode as a distributed constant line and treat the modulation electric signal propagating through the electrode as a traveling-wave, respectively. In that case, in order to increase the effective interaction length with the modulated optical signal and the modulation electric signal which are propagating in the optical phase modulator region, a so-called traveling-wave type electrode which is devised to make a phase velocity vo of the modulated optical signal and a phase velocity vm of the modulation electric signal as close to each other as possible (phase velocity matching) is required.
In order to realize optical waveguide-type semiconductor optical phase modulators and semiconductor MZ optical modulators, a III-V compound semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP), which is useful for forming a light source element, can be used so as to apply materials having a (complex) refractive index with respect to optical signal, which changes as applied electric field changes, to an undoped core layer. In this case, a single-mode optical waveguide with a so-called p-i-n diode structure in which an undoped core layer is sandwiched between a p-type cladding layer and an n-type cladding layer is widely used so as to apply the electric field to the core layer by applying reverse bias voltage.
For example, assume the case where an electrode stripe is provided in a single-mode optical waveguide with a practical p-i-n type diode structure in 1550 nm band mainly used in the optical fiber communication system. In the case of utilizing the optical waveguide as the transmission line of the modulation electric signal, the p-type semiconductor which usually has lower electrical conductivity than the n-type semiconductor has to be used as a cladding layer. Accordingly, the (complex) characteristic impedance (absolute value) of the transmission line, which affects the modulation electric signal, is decreased to about 20Ω, which is less than half of the typical characteristic impedance (50Ω) of microwave circuit components. As a result, this impedance mismatch leads to degradation of the modulation bandwidth due to reflection or the like and increase in driving voltage when the modulation electric signal output from the driving circuit is applied to the optical modulator as a transmission line. For the same reason, the effective refractive index nm (=c0/|vm|, c0: velocity of light in free space) which affects the modulation electric signal is about seven on average. This value is about twice the effective refractive index no (=c0/|vo|, about 3.5 in InP) of the modulated optical signal.
This velocity mismatch between the modulated optical signal and the modulation electric signal limits the effective interaction length therebetween. This leads to degradation of the modulation bandwidth and increase in driving current, as in the case of an impedance mismatch. Thus, in the case of employing the traveling-wave type electrode is employed in the waveguide-type optical phase modulator or the electro absorption type optical intensity modulator, to which the p-i-n type diode structure is applied, there are problems in reduction in operation voltage and increase in bandwidth.
In regard to such problems, it is reported that the phase velocity matching and the impedance matching are attempted to be satisfied by changing the layered structure or the electrode structure of the semiconductor optical modulator. For example, it is reported that the phase velocity matching and the impedance matching are attempted to be satisfied while maintaining a uniform layered structure along the optical signal propagation axis, by employing a layered structure with no p-type semiconductor layer such as an n-SI-i-n type (SI: semi-insulating semiconductor) (Non-Patent Literature 1).
For example, there is proposed a configuration in which a low-impedance region (region in which the phase velocity of the modulation electric signal is low and the characteristic impedance is low) with a p-i-n layered structure and a high-impedance region (region in which the phase velocity of the modulation electric signal is high and the characteristic impedance is high) with an SI-i-n layered structure, for example, are alternately arranged at a sufficiently shorter pitch than that of the propagating wavelength, of the modulation electric signal (Non-Patent Literature 2). According to this configuration, the phase velocity and the characteristic impedance in the both regions are averaged by weighting, thereby satisfying the apparent phase velocity matching and impedance matching.
There is also proposed an optical modulator having a segmented electrode structure in which the electrode of the optical modulator is segmented (Patent Literatures 1 to 3). Additionally, there is proposed a configuration in which the length of each of segmented electrodes arranged in a modulator is a power-of-two multiple of a certain unit length (Patent Literatures 4 to 7).